Cable railways and like systems, especially ski lifts, in which a vehicle or personnel carrier is displaced by a carrying cable strung over a number of pylons from one terminal station to another terminal station are of widespread use at winter sports facilities and the like and are generally equipped with means for halting the operation of the ski lift in the event of a derailment of the carrying cable.
This device comprises, in general, a security unit located at one of the stations of the ski lift, usually the lower station, and provided with a continuous voltage generator and a galvanometer, an aerial safety line (security line) connecting the circuit breakers which respond to derailment of the carrying cable at each of the pylons and which interrupt the circuit, the line running the full length of the ski lift, and an equilibration resistance located usually at the other terminus of the line, generally the upper station, which is usually returned to ground.
By measuring the current flow, the detector unit at the lower station is able to interrupt the operation of the ski lift when one of the circuit breakers, responding to derailment of the carrying cable and serving as a detector thereof, open-circuits and indicates via the aerial line a defect represented by an open-circuit condition or a grounding condition.
Because of the problems prevailing in mountainous regions, for effective use of the system it is necessary to rapidly localize, i.e. locate, the particular defect so that the derailment at the detected location can be remedied and the ski lift placed again in service.
In practice it has been found that the localization of the fault is less precise than is desired with existing apparatus and requires some time to investigate the particular site of the derailment along the line. One localization device currently in use provides resistances of fixed value in parallel with the respective interruptors or circuit breakers so that the needle of the galvanometer has a specific position for the opening of the interrupters. This system has been found to be effective for the detection of cable derailments but is ineffective for the localization of a transient fault, i.e. does not respond in the case of such transient faults.